1. Technical Field
This invention relates to digital filtering systems and, more particularly, to an FIR digital filter having programmable canonic signed digit (CSD) multipliers.
2. Discussion
Digital filters are widely used in various applications for processing digital signals. Digital filters are generally constructed from electronic devices such as adders, multipliers, shift registers, memory units and so on. For applications with narrow transition bandwidths and highly suppressed stopband attenuation, digital filters generally require that many filter taps be implemented in a finite impulse response (FIR) configuration. However, the many filter taps that are required for typical FIR filter configurations, which traditionally have been implemented with full array twos complement multipliers, may drive hardware requirements to such a high level as to become practically unrealizable.
To overcome this problem, a technique referred to as a canonic signed digit (CSD) implementation has been developed which significantly reduces the hardware requirements. A CSD filter implementation employs a special number representation technique which simplifies the hardware required for multiplication operation. Each filter coefficient of a CSD filter is essentially represented as the sum of a small number of powers of two weighted by +1 or -1.
In operation, a CSD filter performs multiplication which is accomplished in hardware with a relatively small number of shifts, negations and additions. This implementation requires far less circuitry than a typical two's complement array multiplier. However, the coefficients for a CSD filter are hardwired to provide a fixed filter which restricts the implementation to a single filter response configuration. As a result, such a filter is configured to provide a fixed filtering response based on fixed filtering coefficients. The fixed coefficients thereby restrict the use of the CSD filter to one particular filtering application which results in an inflexible filter.
It is therefore desirable to obtain a more flexible and compact digital filter which employs a programmable CSD multiplier that allows for a wide range of filter responses. It is further desirable to obtain a programmable and more compact canonic signed digit (CSD) array multiplier.